1. Field of the Technology
The invention relates to a demodulation technology, such as multiuser detection or Rake receiver, in the CDMA cellular mobile communication system; more specifically, to a multiuser detection method or other demodulation method at the receiving side, such as Rake receiver, wherein the method is used in a communication system that has a variable spreading factor at the transmitting end. The method of the invention relates to a technique that indicates the bit of physical code channel and spreading factor for the current frame (currently demodulated frame), such as the Transport Format Combination Indicator (TFCI) technique in a third generation mobile communication system.
2. Related Technology
When designing a cellular mobile communication system, it is considered that how to use the limited radio resources as much as possible and how to provide the subscribers with more and better services.
Comparing with the Rake receiver technique, the multiuser detection technique can raise system performance obviously, and its spectrum efficiency is almost double, so the system capacity is raised. Multiuser detection is divided into two ways: the Interference Cancellation (IC) and the Joint Detection (JD), and either utilizes the user code channel estimation result to demodulate the desired user signal and to eliminate other user signals. Therefore, one pre-condition should be satisfied before using the multiuser detection is that the information about the transmitting side, such as the physical code channel, the spreading factor (SF) and the training sequence etc., need to be known.
Nevertheless, in a real system, especially when a technique that directly indicates the physical code channel and spreading factor of current frame used, such as TFCI in a third generation (3G) mobile system, a transmitted data size changes one frame to another along with the data size generated at the transmitting source. This means that the transmitted data size is non-constant, so the physical code channel and spreading factor used by each user in every Transmission Time Interval (TTI) are changed too, and the receiving side does not know before processing which physical code channel and spreading factor are used at the transmitting side for the user. Usually, it is impossible to use the multiuser detection directly.
One of the solutions is that the receiving side buffers all data sent by the transmitting side in one TTI, and after demodulating the TFCI data of transmitting side, the receiving side demodulates the TTI data. There are four disadvantages of this solution:
1. A large buffer is needed for storing the whole TTI received data;
2. The TFCI data needs to be demodulated first (when the physical code channel and spreading factor are unchanged and known), and it is better to take the multiuser detection to demodulate the TFCI data, otherwise the system performance will be worse because of bad TFCI data demodulation result; so the multiuser detection is used twice: one for the TFCI data and one for the demodulated data, so the computation volume is almost double;
3. The receiving side can demodulate only after a whole TTI data have been received; since the TTI and the physical frame time interval are not equal, for example in a TD-SCDMA system the physical frame time interval is 5 ms and the TTI is possibly 10 ms, the receiving side must wait 10 ms and then makes the demodulation; this will delay the signal processing much longer, and cannot satisfy within the 150 ms delay requirement from terminal to terminal;
4. When directly indicating the current frame physical code channel and spreading factor technique is used, such as TFCI in the 3G, for a variable spreading factor case, it is necessary to look for a blind detection method, and for a fixed spreading factor case, it takes too many bits for indicating the current frame physical code channel and spreading factor, and the net payload bits for transmitting in a frame are too small.
At present, in 3G except TD-SCDMA, other systems use the fixed spreading factor.
The 3GPP has defined the TFCI used in a physical code channel. The TFCI is some physical parameters with very high processing gain, and they are transferred in each TTI to indicate its channel-coding scheme, interleaving and holing pattern etc. With these parameters, the current physical frame spreading factor and the code channel occupied situation can be derived. For example, N code channels have allocated to the users, and among them M code channels are needed to be occupied at an instance, M<=N; from the TFCI data, the demodulation side can derived that M code channels are occupied, and the spreading factor of the M code channels is defined in the standard in advance.
When the wireless link of a user has been established, which physical resources that can be used by this user are defined. Suppose the spreading factor of a user is SF40 (four is the code length of the SF, and the 0th code channel is used), the user can use the spectrum resources drawn from the SF40, which is shown by real lines after the SF40 in the code tree in FIG. 1.
When the user transmits many data, a code channel with spreading factor 4 (SF40) is taken; when a small amount of data is transmitted, one code channel with spreading factor 8 (SF80) and one code channel with spreading factor 16 (SF162) are taken; when a smaller amount of data is transmitted, only one code channel with spreading factor 8 (SF80) is taken; and when a smallest amount of data is transmitted, only one code channel with spreading factor 16 (SF160) is taken. This is the variable spreading factor technique. In each situation mentioned above, which code channel is used is defined in the standard (for description, in this document the code channels shown in above parentheses are taken).
When a smaller amount of data is transmitted, the transmitting side can take larger spreading factor. Since the spreading factor is larger, the processing gain is larger too; so the transmitting power can be smaller. Therefore, on the one hand, the transmitting power is saved (This is useful for a terminal; if voice takes 50% of the transmitting side power, then with this method the transmitting side power can save 25%), and on the other hand, interference to other users is reduced. So, the whole system performance is raised.
It has been seen from the above analysis that the multiuser detection and TFCI techniques are very important. The TFCI indicates the channel-coding scheme, interleaving and holing pattern etc. for the current TTI (10, 20, 40 or 80 ms), but in a physical frame (it is 5 ms for TD-SCDMA) the physical code channel and spreading factor used by a user are unknown, so it is required that the receiver must adaptively detect the variable code channel. Nevertheless, the multiuser detection technique uses the code channel estimation result to make either Interference Cancellation or Joint Detection, but there is a pre-condition that the physical code channel and spreading factor of current frame for all users have been known. Obviously, this is a conflict.